Our invention relates to a system for the conversion of electric signals from digital to analog form. More specifically, our invention concerns a system for translating a digitized audio signal or like digital data signal into an equivalent analog signal with the addition of dither (an artificially created white noise signal) to the digital data signal, possibly with the subsequent removal of the dither from the analog data signal, for the reduction of noise and distortion.
The pulse code modulation (PCM) or digital processing of audio signals has become, or is becoming, the mainstream of high fidelity sound recording and reproduction with the advent and ever increasing commercial acceptance of compact discs (CDs). There are, however, some problems left unsolved in the area of digital sound processing. One of these is the "quantization noise", by which we mean the differences between the samples of the music wave and the quantized values of the samples. The quantization noise becomes particularly pronounced, distorting the reproduced sound as higher harmonics, when the input signal level is low and there are only a small number of quantization steps. Even when the input signal level is high, the quantization noise will distort the signal if it changes slowly.
A conventional solution to this quantization noise problem has been the use of dither, intended to turn the quantization noise into white noise which hardly affects the appreciation of the reproduced sound. A digital dither signal is superposed on the audio signal prior to its digital to analog conversion and may, may not, be removed from the analog audio signal following the conversion. This technique is disclosed for example in the article entitled "The Application of Large Amplitude Dither to the Quantization of Wide Range Audio Signals" by Yoshio Yamasaki in The Journal of the Acoustical Society of Japan, Vol. 39, No. 7, published 1983.
Generated by an analog dither generator, the dither signal must of course be digitized preparatory to its addition to the digital audio signal. Heretofore, if the digitized audio signal is of the 16 bits code, for instance, then the analog dither signal has been digitized by a 16 bits analog to digital converter. Subsequently added to the digital audio signal, the digital dither signal is transformed therewith into analog form by a digital to analog (D/A) converter. We object to the use of the A/D converter having the same number of bits as that of the bits of the digital audio signal for the conversion of the dither from analog to digital form. The cost of the complete digital to analog conversion system will become appreciably lower if there can be employed, for digitizing the analog dither signal, an A/D converter of a smaller bit number than that of the digital audio signal.